This invention relates to an optical information medium such as read only optical disk and optical recording disk.
Optical disks which are recorded and/or read by a light beam such as semiconductor laser beam have become widely popular as an information medium for recording various information including audio and image information.
Most of the optical information media used nowadays are those wherein the information is recorded and/or read in two dimensional manner, and it is not so popular to record a plurality of signals in three dimensional manner, namely, in the depth direction of the information recording layer. Accordingly, increase in the recording capacity needs to be accomplished by increase in two dimensional recording density, namely, by increase in the recording density in the lateral directions of the information recording layer.
The recording density of the main stream optical information medium of today is determined by the beam spot diameter of the laser beam used for the reading of the information recorded in the medium. To be more specific, the recording density can be increased by reducing the beam spot diameter which is proportional to the xcex/NA (wherein xcex is wavelength of the laser beam and NA is the numerical aperture of the objective lens) of the optical system used in the reading. Accordingly, increase in the recording density of an optical information medium needs reduction in the wavelength xcex and increase in the numerical aperture NA. For example, an optical system used with a CD (Compact Disk) has a wavelength xcex of 780 nm and a NA of 0.45 and the one used with a DVD (Digital Versatile Disk) has a wavelength xcex of 650 nm and a NA of 0.60, and as a result, the theoretical minimum readable pit length is 0.43 xcexcm in the case of the CD and 0.27 xcexcm in the case of the DVD. In the near future, the semiconductor laser which is used as the light source is likely to be the one having a shorter wavelength, namely, the one in blue/violet region or ultraviolet region to realize further improvement in the recording density.
An optical disk typically has a structure wherein an information recording layer is formed on a light-transmitting substrate, and the recording and/or reading laser beam reaches the information recording layer through the light-transmitting substrate. Accordingly, it is desirable that the light-transmitting substrate is formed from a material which exhibits minimum possible loss of the light that passes therethrough by absorption.
The semiconductor laser which is commonly used for the light source inevitably suffers from variety in the oscillation wavelength from product to product, and this variation is normally within the range of about 10 to 20 nm. The oscillation wavelength also fluctuates depending on the exterior temperature even for the same product, and for example, the oscillation wavelength may fluctuate within the range of about 10 nm when the temperature is in the range of 0xc2x0 C. to 60xc2x0 C. The light-transmitting substrate, therefore, should exhibit a high light transmittance at the wavelength of the recording/reading beam as well as no significant change in the light transmittance at and near the wavelength of the recording/reading beam. It is also necessary to select a material which undergoes no substantial change in the light transmittance over the long period for the light-transmitting substrate.
The light-transmitting substrate typically comprises a thermoplastic resin such as polycarbonate or polymethyl methacrylate (PMMA) in view of the excellent moldability and high transparency as well as the low cost. These resins exhibit good transparency to the beams of infrared or red region which are used as the recording/reading beams for the current commercial optical disks.
Increasing the NA of the objective lens, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of a medium relative to the optical axis of the optical system, which depends on the numerical aperture NA. The tilt margin is in proportion to
xcex/(txc2x7NA3)
wherein xcex denotes wavelength of the recording/reading beam and xe2x80x9ctxe2x80x9d denotes thickness of the light-transmitting substrate through which the recording/reading beam enters the medium. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2xcex8)xe2x88x925/2
wherein n denotes the refractive index of the substrate and xcex8 is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness xe2x80x9ctxe2x80x9d of the light-transmitting substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
A structure enabling further decrease in the light-transmitting substrate thickness has been proposed in order to enable use of an objective lens of larger numerical aperture NA. In this structure, a substrate having a thickness of about 1 mm is used as a supporting substrate for ensuring rigidity of the medium, and the pits and the recording layer are formed on its surface, and a light-transmitting layer in the form of a thin substrate having a thickness of about 0.1 mm is formed on the recording layer. The medium is irradiated with the recording/reading beam through this light-transmitting layer. This structure enables drastic reduction in the thickness of the substrate, and high density recording by the use of a higher NA is thereby enabled. A medium having such structure is described, for example, in Japanese Patent Application Laid-Open Nos. (JP-A) 320859/1998 and 120613/1999. Provision of the light-transmitting layer with the thickness of about 0.1 mm enables an objective lens having a large NA of, for example, about 0.85.
Polycarbonate, PMMA, and other thermoplastic resins which had been typical materials for use in producing the light-transmitting substrate normally undergo decrease in the transparency as the wavelength of the light source becomes shorter, and these materials exhibit rapid increase in the absorption when the wavelength gets as short as about 400 nm or shorter. Therefore, use of such resins for the light-transmitting substrate is unfavorable in the optical disks wherein a beam in the blue to ultraviolet region is used for the recording and/or reading.
These resins also experience secular deterioration caused by the UV-containing light such as sunlight, and as a result, absorption increases at a wavelength shorter than about 400 nm. It is quite difficult to realize stable recording/reading properties for prolonged period by using such resin when a light in the region of blue to ultraviolet is used for the recording/reading.
Metal oxide layers such as the one comprising SiO2 and the like are known to be a film which exhibits good transparency in a short wavelength region of approximately 400 nm. A metal oxide layer, however, is usually formed by a thin film-forming process such as evaporation or sputtering and the metal oxide layer can not be used for a light-transmitting substrate which needs a thickness of at least about 0.1 mm. A process which requires a vacuum such as evaporation or sputtering is also inevitably associated with increase in the scale of the installation and complication of the production process as well as problem of an increased cost.
The present invention has been completed in view of the situation as described above, and an object of the present invention is to provide an optical information medium provided with a light-transmitting substrate which exhibits excellent light transmittance when a laser beam of blue/violet region or ultraviolet region is used for the recording and/or reading beam; whose light transmittance is not substantially affected by the variation or fluctuation in the oscillation wavelength of the laser beam source; which has a long-term weatherability; and which exhibits sufficient protective effects against exterior impacts.
Such objects are attained by the present invention as described in (1) to (7), below.
(1) An optical information medium comprising an information recording layer and a light-transmitting substrate, which is used such that the laser beam for recording and/or reading reaches said information-recording layer through said light-transmitting substrate, wherein
said light-transmitting substrate has a light transmittance T and difference xcex94T between the maximum value and the minimum value of the light transmittance T in an arbitrary wavelength range of 10 nm which satisfy the relations:
Txe2x89xa70.8,
xcex94Txe2x89xa60.10
at least in the wavelength region of 250 to 450 nm.
(2) An optical information medium according to the above (1) wherein the relations:
Txe2x89xa70.8,
xcex94Txe2x89xa60.10
are satisfied at least in the wavelength region of 200 to 450 nm.
(3) An optical information medium according to the above (1) wherein the relations:
Txe2x89xa70.8,
xcex94Txe2x89xa60.10
are satisfied at least in the wavelength region of 250 to 450 nm after 500 hour exposure to laboratory light used in the methods of exposure to laboratory light sources defined in JIS K7350-2-1995.
(4) An optical information medium according to the above (1) wherein said light-transmitting substrate includes at least one resin layer, and at least one layer of said resin layer(s) contains a fluorohydrocarbon-based resin.
(5) An optical information medium according to the above (1) wherein
said light-transmitting substrate has a surface layer which constitutes the beam incident surface of the medium and an inner layer between said surface layer and said information-recording layer;
said inner layer is a resin layer containing a fluorohydrocarbon-based resin;
said surface layer comprises an organic compound other than said fluorohydrocarbon-based resin and/or an inorganic compound; and
said surface layer has a thickness less than that of said inner layer.
(6) An optical information medium according to the above (4) wherein said resin layer containing the fluorohydrocarbon-based resin is amorphous.
(7) An optical information medium according to the above (1) wherein the light-transmitting substrate has a thickness of 5 to 300 xcexcm.